var arr = [1, 2, 3]
arr.a = 123
Array.prototype.a = 123

for (let value of arr) {
    console.log(value)
}
// console.log(arr);
// for (const iterator of object) {

// }
// for of 遍历数值
// for in 主要遍历下标，而且不可以加减

var arr1 = [1, 2, 3, 4]
for (let index in arr1) {
    // console.log(index + "***");
    let res = arr1[index]
    console.log(res)
}